Cutting elements, as for example cutting elements used in rock bits or other cutting tools, typically have a body (i.e., a substrate), which has an interface end or surface. An ultra hard material layer is bonded to the interface surface of the substrate by a sintering process to form a cutting layer, i.e., the layer of the cutting element that is used for cutting. The substrate is generally made from a tungsten carbide-cobalt alloy (sometimes referred to simply as “cemented tungsten carbide,” “tungsten carbide” “or carbide”). The ultra hard material layer is a polycrystalline ultra hard material, such as polycrystalline diamond (“PCD”), polycrystalline cubic boron nitride (“PCBN”) or a thermally stable product (“TSP”) material such as thermally stable polycrystalline diamond.
Cemented tungsten carbide is formed by carbide particles being dispensed in a cobalt matrix, i.e., tungsten carbide particles are cemented together with cobalt. To form the substrate, tungsten carbide particles and cobalt are mixed together and then heated to solidify. To form a cutting element having an ultra hard material layer such as a PCD or PCBN ultra hard material layer, diamond or cubic boron nitride (“CBN”) crystals are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure (e.g., a niobium enclosure) and subjected to high temperature and high pressure so that inter-crystalline bonding between the diamond or CBN crystals occurs, forming a polycrystalline ultra hard diamond or CBN layer. Cobalt from the tungsten carbide substrate infiltrates the diamond or CBN crystals and acts as a catalyst in forming the PCD or PCBN. A catalyst material may also be added to the diamond or CBN particles to assist in inter-crystalline bonding. The process of high temperature heating under high pressure is known as high temperature high pressure sintering process (“HTHP” sintering process). Metals such as cobalt, iron, nickel, manganese and alike and alloys of these metals have been used as a catalyst matrix material for the diamond or CBN.
In some instances, the substrate may be fully cured. In other instances, the substrate may be not fully cured, i.e., it may be green. In such case, the substrate may fully cure during the HTHP sintering process. In other embodiments, the substrate may be in powder form and may solidify during the sintering process used to sinter the ultra hard material layer.
TSP is typically formed by “leaching” the catalyst (such as the cobalt) from the polycrystalline diamond. This type of TSP material is sometimes referred to as a “thermally enhanced” material. When formed, polycrystalline diamond comprises individual diamond crystals that are interconnected defining a network structure. A cobalt binder phase (i.e., the catalyst) is found within interstitial spaces in the diamond network, between the bonded diamond crystals. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond, and as such, upon heating and/or cooling of the polycrystalline diamond during use, the cobalt expands, causing cracks to form in the diamond network, resulting in the deterioration of the polycrystalline diamond layer. In addition, during use, the catalyzing effect of the cobalt can cause graphitization in the interstices of the diamond network, which deteriorates the diamond. By removing, i.e., by leaching, the cobalt from the diamond network structure, the polycrystalline diamond layer becomes more heat resistant. In another exemplary embodiment, TSP material is formed by forming polycrystalline diamond with a thermally compatible silicon carbide binder instead of cobalt. “TSP” as used herein refers to either of the aforementioned types of TSP materials.
To reduce the residual stresses created at the interface between the substrate and the ultra-hard layer, prior art interface surfaces on substrates have been formed having a plurality of projecting spaced apart concentric annular rings, such as annular ring 5 shown in FIG. 1A. Due to the difference in the coefficients of thermal expansion of the substrate and the ultra hard material layer, these layers contract at different rates when the cutting element is cooled after HTHP sintering. Tensile stress regions 2 are formed on the upper surfaces of the rings 3, whereas compressive stress regions 4 are formed on the valleys between such rings, as shown in FIG. 1B, which shows a cross-sectional view of a projecting ring. Consequently, when a crack begins to grow it may grow annularly along the entire upper surface of the annular ring where it is exposed to tensile stresses, or may grow along the entire annular valley between the projections where it is exposed to compressive stresses, leading to the early failure of the cutting element. In other prior art cutting element substrate interfaces incorporating spaced apart projections, the projections have relatively flat upper surfaces or non-planar upper surfaces having one or more shallow depressions. Applicants believe that such upper surfaces may allow a crack to grow and gain momentum and thus become critical.
Common problems that plague cutting elements are chipping, spalling, partial fracturing, cracking and/or exfoliation of the ultra hard material layer. Another frequent problem is cracking on the interface between the ultra hard material layer and the substrate and the propagation of the crack across the interface surface. These problems result in the early failure of the ultra hard material layer and thus in a shorter operating life for the cutting element. Accordingly, there is a need for a cutting element having an ultra hard material layer with improved cracking, chipping, fracturing and exfoliating characteristics, and thereby having an enhanced operating life.